Add correlation_id to TransactionCreateRequest and TransactionHeader

Callers can set correlation_id when creating transactions to link
related transactions together (e.g. all transactions in a workflow).
The field propagates through the PRIME pipeline and child transactions.

models/models.go

@@ -9,6 +9,7 @@ type TransactionCreateRequest struct {
 	TxnType       string `json:"txn_type"`
 	Payload       string `json:"payload"`
 	Tag           string `json:"tag,omitempty"`
+	CorrelationId string `json:"correlation_id,omitempty"`
 }
 
 type TransactionCreateResponse struct {
@@ -42,6 +43,7 @@ type TransactionHeader struct {
 	BlockId       string `json:"block_id"`
 	TxnType       string `json:"txn_type"`
 	Timestamp     string `json:"timestamp"`
+	CorrelationId string `json:"correlation_id,omitempty"`
 }
 
 type TransactionProof struct {